Lasers have found utility in the treatment of skin lesions such as port wine stains and telangiectasia. They exert their effects on tissue when the high-intensity incident beam of photons is absorbed by a pigment, chromophore, with an appropriate absorption spectrum, releasing heat at the site of this photo-thermal interaction.
Laser wavelengths that are absorbed by target tissues, with the intent to leave surrounding tissues unaffected, improve the specificity of laser treatments. For treatment areas with complicated geometries one technique is to irradiate a large area of the lesion. Tissue absorption characteristics are used to deposit the laser energy in the desired location. This is, however, difficult because the difference in absorption between the target and surrounding tissue varies with each patient. An alternative is for the physician to track the target by following the outline of the treatment area by hand, thereby leaving the surrounding tissue unaffected. Some targets are too small to be lased by such a manual tracking technique. Complicated scanning mechanisms and impractical robotic schemes have been proposed and built to facilitate the laser treatment process. These have proven to be too expensive and clumsy to use.
There are commercial products which deliver laser energy to an area to be treated. Such devices usually indiscriminately deliver laser energy to an area regardless of the need for treatment. This is determined visually by the physician. Discrimination between contrasting dermatological tissue is achieved visually and often is imprecise.
Cutaneous spectrophotometry, a technique that measures reflected monochromatic light from skin, has been employed as a noninvasive method to characterize in vivo pigments on port wine stains, S. V. Tang, et al., The Journal of Investigative Dermatology, 1983, 80, pages 420 to 423.
Port wine stains have been treated with yellow 578 nm light from a copper vapor laser. In one instance light was applied by scanning a 1 mm optical fiber approximately 2 mm above a lesion. A maximum scan rate of 3s/cm2 was used, J. W. Pickering, et al., British Journal of Plastic Surgery, 1990, 43, pages 273 to 282. Each patient was assigned to a particular class of vascular abnormality. The periphery of the area to be treated was marked with a green pen which contrasted with the color of the lesion. The green outline provided a finishing point for the scan because the true edge of the lesion was not easily discerned.
The color of healthy skin is determined largely by the quantity and degree of oxygenation of blood in the dermis and the presence or absence of the brown/black epidermal pigment, melanin. A specially designed skin reflectance spectrophotometer, the Haemelometer, has been developed for the quantification of cutaneous hemoglobin and melanin by Feather et al., Phys. Med. Biol., 1988. Vol. 33, No. 6, pages 711 to 722. The Haemelometer consists of a power supply, an electronic drive with signal processing capability and a skin reflectance measuring head. Nine LED's and a silicon photodiode detector are positioned in a hollow hemisphere measuring head. Reflectance signals generated in the silicon photodiode detector are amplified and separated into three channels, corresponding to each of three wavelengths. The measuring head is held lightly over an area of skin being studied and provides simultaneous measurement of the hemoglobin and melanin indices.
None of the preceding devices or methods provide a dermatology handpiece which establishes a threshold signal to differentiate between normal skin tissue and a lesion in such a manner that a treatment beam of optical energy is delivered only when the threshold signal is exceeded. Current devices do not provide selectivity between normal tissue and the lesion, other than by visual inspection. Such devices deliver optical energy to lesions and normal tissue without distinguishing between the two.